Hi everyone,Richard EI5GUB here,Ive just put a 20 mtre loop up and fed it into a 4:1 balun,it shows 50 ohms at 14.185 but has an SWR of 3 to 1,how can I get this down,also when I removed the Balun and fed direct to the Anaylser it shows an impedeance of around 75 to 80 ohms which probably puts a 4:1 balun out of the picture,the antenna was 4 feet off the ground when tested and the cable from balun to anaylser was 1 foot longim planning a complete circular loop if I can get this delta working right,any advice please,cheers Richard Gorman.

Depending on several different factors the impedance of a full wave loop is 80 - 100 ohms. You would do better by feeding it directly with 50 ohm coax instead of the 4:1 balun. Better yet feed it thorugh a 1/4 wave coax matching section.

The 1/4 wave (electrical length, accounting for velocity factor) per AD4U's suggestion is easiest but does not provide the decoupling a balun would.

The other way is to feed it through a 2:1 balun. They are available (at least here in the States) from a couple of commercial sources.

2:1 baluns are quite handy....you can feed dipoles at electrical elevations where the Z is near 90 ohms and they work well on Loops which are about 100 ohms Z.

If you want low SWR, your coax must see something CLOSE to its 50 ohm impedance. A 4:1 balun would theoretically multiply the 50 ohm coax impedance to about 200 ohms, so you then have a 200 to 100 mismatch..... so it is not what you want.

Hi K0ZN,thanks for replying,yes I was thinking a 2 to 1 would do the job but they are impossible to get here in Ireland and I would build one no problem if I could get some instructions,9 6 and 4 to 1 seem quiet common but 3 to 1 or 2 to 1` seem scarce,im putting this loop up at 70 feet pointing north west /south east out of central Ireland so will be good for USA and down into Asia

How are you measuring the antenna? A 50 ohm antenna can't have an SWR of 3 : 1 unlessthere is significant reactance. If that is the case, then adjusting the wire length shouldbring the SWR to a reasonable value. (Or inserting a coil or capacitor in series with thewire at the feedpoint.) But this may not be the case if you are measuring the antennathrough any significant length of coax.

Loops can have a wide range of input impedances depending on the shape. If you makeit rectangular and about twice as tall as it is wide, then feeding it in the middle of thebottom wire gives very close to 50 ohms. If you spread the sides out so it is wider thanit is tall then the impedance will be closer to 200 ohms (with slightly lower gain and widerSWR bandwidth than the 50 ohm version.) The same approaches apply to delta or diamondshapes.

Quote

...the antenna was 4 feet off the ground...

Antenna impedance varies with height above ground. I would guess that the resistance willincrease and the resonant frequency will shift up the band by perhaps 100kHz when theantenna is raised into the air. Getting the bottom wire 5m off the ground will give you a better sense of what it will be at the final height. (This also depends on whether you areusing a single loop or a quad, as the parasitic elements of the latter will increase thevariation.)

I suspect that the elongated rectangle will end up giving you more gain than a circularloop and a more convenient feedpoint impedance (besides being easier to construct.)Here are some references:

(Yes, they require a free registration, but this is one of the best antenna resourceson the internet.) The circular loop can give about 0.3dB improvement over a square,while the elongated loop is about 1dB better.

Generally I find that I can get close enough to match any full-wave loop using either direct feed (50 or 75 ohms), a quarter wave of 75 ohm transmission line, or a 4 : 1 balun, depending on the shape of the loop.

I am guessing this is a vertically oriented 20M loop? A horizontal full wavelength loop at any height has the majority of its energy going straight up- not what one would generally want for 20M DX. At low angles a high full wave loop would have a pattern about like a dipole. So if this loop is horizontal, put up a dipole, you'll be much better off for DX.

Hi WB6BYU thanks for alot of info,I was using 1 foot of coax to analyser,the loop is triangular and is equal sided,7 mtrs a side and was tested horizontal,I will have to find a 1/4 wave piece of 75 ohm coax and try it.

yes Dale it is vertical and will be 70 feet high between 2 big fir trees,I know you can feed them at different points so they are either vetical or horizontal,but will stick with bottom corner,Iwill stick with a triangle loop rather than a circular one as it seems they have not much gain

...the loop is triangular and is equal sided,7 mtrs a side and was tested horizontal...

That will be quite different than what you will see when it is up in the air due to thecoupling to the ground. You really can't take good measurements for either impedanceor resonant frequency unless it is vertical and some distance above the ground.

I've never figured out what that "resistance" scale reads on some analyzers when theload has reactance, as it must in this case (otherwise a 50 ohm load would have a 1 : 1SWR.) Even 1' of coax can shift reactance measurements, though it shouldn't affectthe SWR curve. If you can find the point of minimum SWR that will give you a good ideaof the resistive component of impedance.

Since you have the 4 : 1 balun on hand you can still use it. Find the SWR at minimum andfrom that determine the input impedance (let's say the minimum SWR with the 4 : 1 balun is 2 : 1, so the input impedance of the loop is 100 ohms.) Then you can use a beta match(shorted stub or shunt coil) across the balun to raise 100 ohms to 200 ohms, along with abit of adjustment to the loop length. W4RNL's matching spreadsheet suggests that about1m of shorted 600 ohm ladder line or a 2.25uH inductor would give a good match on 20m.That's probably easier to find than a fractional ratio balun (which I find suspect anyway.)

But it's probably easier simply to make the delta loop wider so it provides a 200 ohm match.

Another thought: I saw in a very old Antenna book of some sort where the full wave loop was TWO turns and fed with balanced line. I don't have data on thisand/or what the feedpoint impedance would be. You may want to research this some. I have not looked into the math to see what a second turn woulddo to the input impedance....or even if this is a viable idea.

hi i got some 75 ohm coax and am making a 1/4 wave matching section,when I have it connected to the antenna and go analysing will I see 50 ohms at the end of the 1/4 wave or does it need to fed into 50 ohm cable also and then then checked

If it is 50 ohms at the end of the matching section it should still be 50 ohms even witha length of 50 ohm coax attached. Assuming you mean 50 + j0 ohms, that is.

Pay attention to the SWR rather than the R meter reading. When the SWR is highthen the R value will vary significantly with line length. It's the frequency of minimumSWR, and the SWR at that point, that you want to work with. If the frequency ofthe SWR dip is outside the band, adjust the wire length to put it where you want it.If the SWR at the minimum is too high, adjust the loop shape to improve it.

Using 2mm wire it looks like 7m on a side is a bit short. About 22 to 22.5m total length looksbetter, but that will require some adjustment if you are using insulated wire. If you make thetop section 7.5m long then you can adjust the lengths of the other two sides at the feedpoint.

Feed it with 4m of 75 ohm coax (velocity factor = 0.8, typical of foam dielectric. Use 3.3mif it has a solid dielectric. +/- 25cm will still give good results.) SWR bandwidth is about500kHz at 1.5 : 1, so no problem covering the whole band.

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